(1) Field of the Invention
The present invention relates generally to a method and apparatus for differentially softening plastic for use in packaging of articles.
(2) Description of Related Art
U.S. Pat. Nos. 4,281,502, 4,688,367 and 5,123,228 to Bonkowski disclose various plastic carriers which package a plurality of articles such as beverage cans with a single sheet of thermoplastic material.
To form the carrier, selected areas of the sheet are first softened and then drawn down over the tops of the articles. When the sheet rehardens, the articles are firmly bound together. Such a carrier has been found to have a number of advantages. For example, the carrier can be designed to completely cover the tops of the articles which can protect the articles from dirt and contamination.
Typically, the portion of the sheet which is directly over the top of each article is selectively softened by the application of heat. U.S. Pat. Nos. 4,688,367 and 5,123,228 disclose that the heat may be applied by a heated platen or button Which either contacts the areas or is brought into close proximity. U.S. Pat. No. 4,281,502 discloses that the heat may be applied by a radiant heater in conjunction with an insulating plate which acts to direct the heat to the appropriate areas. The heat transferred to the sheet softens the areas to permit subsequent stretching.
In both of these methods, the insulating plate and the heated platen should be carefully positioned relative to the sheet to ensure that the softened areas of the sheet are correctly located. A requirement for precision registration of the insulating plate or the heated platen to the thermoplastic sheet can increase the complexity and hence the expense of the packaging process, particularly in high speed operations.